The classical procedure for making metal oxide varistors such as zinc oxide varistors involves dry blending metal oxides and metal salts with zinc oxide powder. Water is than added to form a slurry and the slurry is then pan-dried while heating and stirring to attain a uniform distribution. The dried mixture is ground, sieved, calcined, crushed, sieved, re-made into a slurry with H.sub.2 O, a binder added, the mixture ball-milled (in order to reduce particle size and again obtain uniform distributions), pan-dried once more, crushed and again sieved. The powder is then ready for forming into appropriate configurations before sintering.
An intrinsic difficulty with this classical procedure of blending the metal oxide/metal salts with ZnO powder relates to the tremendous ranges in particle size, morphology, surface area, and chemical reactivity that have to be reconciled in order to achieve a uniform blend of all ingredients. It is usually necessary to utilize drastic mechanical means in order to effect a uniform blend (i.e. ball mills, grinders, pulverizers, etc.). Other non-mechanical means have included the selection of additives with more closely matched physical features (particle size, particle morphology, etc.) and rheological features (handling). This approach is difficult to realize because of the lack of sources from which such uniformity between radically different chemicals can be realized. Another technique has included the preconcentration of all or some of the additives by calcination and a portion of this preconcentrate is then treated as a single or major additive to the ZnO bulk.
The need to produce a homogeneous metal oxide varistor powder is great. The property of homogeneity is very important both in varistors for low voltage applications and for high voltage applications.
This homogeneity promotes uniform electrical properties due to uniform grain size; therefore, exceptional resistance to degradation when suppressing voltage surges can be obtained.